Reactors, such as CANDU, employing heavy water moderation are attractive because of their ability to use natural uranium as the feed fuel. The lattice pitch needed for such D.sub.2 O reactors, however, is usually large as compared to other types of reactors using enriched fuels, resulting in a big core and large inventory of the heavy water moderator which is quite costly. It is recalled that a significant part of the capital cost of a heavy water reactor is for the D.sub.2 O inventory.
As for light water reactors, it is well known that only enriched uranium fuels can make the reactor critical. However, if the uranium enrichment of the fuel can be reduced without decreasing the overall neutron multiplication effect, the fuel cost of this type of reactors will be more economic than that of the existing ones.